Constant-folding missing for ctz#/clz#/popCnt#

Constant-folding is missing for ctz#/clz#/popCnt# primops.

added Pnormal Tfeature request labels

mentioned in merge request !6532 (closed)

mentioned in commit 4115109e

mentioned in commit d41cfdd4

Added in d41cfdd4

closed

unordered-containers will profit from this. It would be great if this could be backported to 9.2.

EDIT: Actually, this feature is probably not very important for u-c, just nice to have.